What lesson does the Mariner learn from his experiences?
The ancient mariner learned two things as a result of his harrowing supernatural experience. He learned to value human companionship, and he learned to love both people and animals.
At the beginning of the tale the mariner relates in the poem, the mariner is one of fifty-one sailors on a ship. At first he seems one with the others; they cheer the albatross when it appears and appreciate the luck it seems to bring them. But something causes him to separate from his fellows when, instead of offering food or playing with the albatross like the other mariners, he shoots the bird with his crossbow. At first the men are upset with him, but then they change their tune and believe that the bird had brought "the fog and mist" and that he was right to slay it. However, when the ship gets stuck in the doldrums, the men hang the albatross around his neck, showing he is no longer accepted by them but blamed. When all the men except the mariner fall down dead, their eyes curse him. He endures their cursing looks for seven days and comes to realize that curse is "more horrible" than an orphan's curse. He seems to wish he could die rather than be "alone, alone, all, all alone." Only he and "a thousand thousand slimy things lived on."
The point at which he blesses the water snakes is the turning point. He is able to pray, and the rain comes and the spirits sail him back home. When he returns, he must continue to do penance by sharing the lesson he has learned with chosen listeners. When summing up his tale to the Wedding-Guest, he describes his loneliness and declares that "sweeter than the marriage-feast" is walking "together to the kirk with a goodly company!" In other words, he has learned to value human companionship. Additionally, he tells the guest, "He prayeth well, who loveth well both man and bird and beast." By this he means that God's favor rests on the person who loves not only humans, but also all the creatures God has made. God loves the people he has made, so his people should love God's creation as well.
The lessons the mariner learned, which he passes on to the Wedding-Guest, are first that nothing is as sweet as good human fellowship and second that people should love "all things both great and small" in God's creation.
The mariner does learn the consequences of his unthinking action (killing the albatross) through the death of his fellow shipmates. He is brought on the road to redemption by the "unthinking" blessing of the watersnakes, creatures he before had thought abhorent. When he does this the albatross which the other sailors had hung about his neck falls from him. The sailors arise, but the are not resurrected and themselves again. They are possessed, in a sense, by angels. These angels use the bodies of the sailors to get the mariner back home. Though the mariner does learn to appreciate nature as a result of his experiences, the consequences of his unthinking action are not erased. The sailors are still dead and go down with the ship when they reach harbor. And the mariner still has penance he must do. He is compelled to tell his story to people that he meets. He does not tell everyone, however. The compulsion to tell his story comes over him at unanticipated times. The people to whom he tells the story must learn from it to appreciate all of God's creation, just as the mariner has learned.